This invention relates to starting of high intensity discharge lamps and, more particularly, to methods and apparatus for starting of discharge lamps containing noble gases at pressures in excess of 300 torr.
High intensity discharge lamps such as high pressure sodium lamps commonly include noble gases at pressures below 100 torr. Lamps containing noble gases at pressures below 100 torr can be started and operated by utilizing an igniter in conjunction with a lamp ballast. The igniter provides high voltage, short duration pulses which assist in initiating discharge. The lamp ballast converts the ac line voltage to the proper amplitude and impedance level for lamp operation.
It has been found that the inclusion in high pressure sodium lamps of xenon as the noble gas at pressures well in excess of 100 torr is beneficial to lamp performance. However, the igniter described above does not produce reliable starting at xenon pressures above about 100 torr. A conductor wrapped around the discharge tube and connected to one of the electrodes is described as having been utilized in assisting the starting of a lamp containing xenon at pressures up to 300 torr in U.S. Pat. No. 4,179,640, issued Dec. 18, 1979, to Larsen et al. The lamp is described as having been operated from a conventional ballast and starting pulse generator.
Another arrangement for starting high pressure discharge lamps is shown in U.S. Pat. No. 4,137,483, issued Jan. 30, 1979, to Ochi et al. A switching circuit contained within the lamp induces a high voltage starting pulse. The high voltage pulse operates in conjunction with a conductor wrapped around the discharge tube to initiate discharge in the lamp. The igniter and the conventional ballast are not used.
Recent developments have indicated the desirability of including xenon at pressures in excess of 300 torr in high pressure sodium lamps. However, none of the starting arrangements described above are effective to reliably start lamps having xenon pressures in excess of 300 torr.